1. Field of the Invention
The present invention relates to transmission lines having multiple I/O (input/output) devices attached thereto, and, in particular, to methods and apparatuses for reducing transmission line noise caused by state transitions of the I/O devices.
2. Description of the Related Art
Dozens or more electronic components such as chips containing integrated circuits (ICs) are often coupled via I/O (input/output) terminals to the same transmission line or bus, such as a "backplane" bus of a computer system. The chips are typically coupled to the bus by an output buffer (also sometimes referred to as a bus driver, which comprises a bus driver transistor such as an n-mos field-effect transistor). Each point of interconnection, such as at the junction of one of these ICs and the bus or at a board-edge connector within the bus, introduces an impedance "discontinuity" along the length of the bus that can give rise to reflections along segments of the bus between these discontinuities. Because of the propagation delay between discontinuities, and other transmission line characteristics of the bus, when the termination points of these segments are not ideally matched, these bus structures will exhibit eigenstates or modes from which resonances will arise. If the signals applied to these buses contain energy at frequencies commensurate with these eigenstates or modes then the resonances will be excited.
It is desirable for the output buffers to transfer information at high rates; however, sharp state transitions comprise energy at yet higher frequencies which do not contain information germane to the data being transmitted and which can excite the typically high resonant frequencies of the bus, thus giving rise to excessive noise. A state transition occurs when the output buffer causes the bus voltage to change from a first to a second state voltage, where the first state voltage corresponds to one of a high or low state voltage, and the second state voltage corresponds to the other of the high or low state voltage. In other words, if a high state corresponds to 3.3V, and a low state corresponds to approximately 0V (ground), then a state transition occurs when the output buffer changes the bus voltage from 3.3V to 0V, or from 0V to 3.3V.
For example, high-performance CMOS (complementary metal oxide semiconductor) ASIC (application-specific integrated circuits) designs with hundreds or more I/O terminals on a single chip are becoming common. Wide (e.g., 72 bit) high-speed signal buses are often used to interconnect VLSI (very large scale integration) components. Conventional unterminated interconnects for CMOS-level signals usually have poor signal quality with severe overshoot and ringing, accompanied by electromagnetic interference (EMI) and a tendency to trigger latch-up.
ECL (emitter-coupled logic) is a very high speed logic family that, rather than attempting to swing potentials from rail to rail, mitigates the difficulties of charging and discharging capacitive node parasitics by steering current (in each gate) alternately through one or the other of two paths. ECL-based high-performance systems have used terminated transmission line interconnects to avoid noise such as ringing and reflections. Such techniques are discussed in Blood, W. R., "MECL System Design Handbook," Motorola Semiconductor Products. However, ECL implementations have a relatively high power dissipation and typically require additional power supply levels and differential signal paths.
Another solution to this problem of transmission line noise is described in Bill Gunning, Leo Yuan, Trung Nguyen, and Tony Wong, "A CMOS Low-Voltage-Swing Transmission-Line Transceiver," IEEE 92CH3128-6/92/0000-0058 (ISSCC 92, Session 3, High-Performance Circuits, Paper WP 3.7). In this approach, sometimes referred to as GTL (Gunning transistor logic), the gate driver places momentary DC feedback around the output driver FET (field-effect transistor) to make the stepwise transition from one output state to another. Power dissipation is reduced if the signal voltage swing is small, but the minimum voltage swing must be large enough to assure acceptable noise margin. The objective of using GTL is to limit the bandwidth of the signal to frequencies below the resonances that invariably crop up in such systems, and hence minimize noise and ringing (generically referred to herein as noise). With the ever-faster circuit speeds (state transition times and propagation delays) of more complex and/or advanced systems that are emerging, however, conventional GTL-type approaches may still exhibit unacceptable noise and ringing.